A wavelength multiplexed light signal network which employs OADM devices using a wavelength division multiplex (WDM) technique are placed in operation under present circumstances in accordance with a path setting in which the network is closed within a ring which consists of about several to more than ten OADM devices. However, two or more rings of this type exist actually, and a wavelength multiplexed light signal network is configured in a form in which the two or more rings are connected in a row. Therefore, there will be a demand for a larger path setting in which a wavelength multiplexed light signal network is placed in operation across a plurality of rings, and attention is given to a wavelength select switch (WSS) as a means for implementing this demand.
On the other hand, in a wavelength division multiplex device, such as an OADM device, an optical demultiplexing unit which demultiplexes a light signal and an optical multiplexing unit which multiplexes light signals are connected to each other via an optical patch cable, and an optical multiplexing/demultiplexing unit which consists of the optical demultiplexing unit and the optical multiplexing unit and an interface unit for connecting with another device are connected to each other via an optical patch cable. However, because the system is based on the premise that a connection job of connecting between units using such an optical patch cable is done manually by a worker, the detection of a connection failure caused by an erroneous operation in the connection job is a very important issue at the time of quick construction, at the time of restoring the system to proper working condition when an emergency failure is occurring, etc.
Because when the optical multiplexing/demultiplexing unit wavelength-demultiplexes a wavelength multiplexed light signal into light signals having individual wavelengths, or inversely multiplexes light signals having individual wavelengths into a wavelength multiplexed light signal, the transmission of the light signals is carried out only in a single direction, there is a one-to-one correspondence between the transmitting ports of the optical demultiplexing unit and the receiving ports of the optical multiplexing unit, and this correspondence is determined beforehand and mechanically in the system. Therefore, the worker should just connect an optical patch cable between two transmitting and receiving ports which are determined mechanically without errors.
Such an OADM device detects and compares the light level of a light signal transmitted from a transmitting port of an optical demultiplexing unit thereof, and the light level of a light signal received at a receiving port of an optical multiplexing unit thereof with each other to detect a connection failure occurring in an optical patch cable. More specifically, after connecting between a transmitting port a and a receiving port a by using an optical patch cable a first, for example, the light level of a light signal at the transmitting port a and the light level of a light signal at the receiving port a are detected and compared with each other. When both the light levels have appropriate values, it is determined that the optical patch cable a is connected normally between the transmitting port and the receiving port, whereas when both the light levels do not have appropriate values, it is determined that a connection failure occurs in the optical patch cable a for connecting between the transmitting port and the receiving port. Then, the next transmitting port b and the receiving port b are connected to each other by using an optical patch cable b, and the light level of a light signal at the transmitting port b and the light level of a light signal at the receiving port b are detected and compared with each other to determine the connection state of the optical patch cable b. After that, a process of detecting a connection failure is carried out on all remaining optical patch cables in the same way (for example, refer to patent reference 1).
However, because when an optical multiplexing/demultiplexing unit has a wavelength select switch, the optical multiplexing/demultiplexing unit can determine whether or not to have no choice but to wavelength-demultiplex a wavelength multiplexed light signal input thereto into light signals having different wavelengths, the OADM device can be set up to transmit two or more light signals having different wavelengths (n transmission signals/n reception signals) via each port. In this setup, arbitrary ports can be specified and therefore a light signal can be transmitted via many paths. Therefore, ports which are connected to each other by using an optical patch cable are not determined uniquely and mechanically, and therefore a connection failure occurring in an optical patch cable cannot be detected only through the detection and comparison of the light levels mentioned above.
To solve this problem, in an OADM device having such a wavelength select switch, each light amplifier is used to piggyback the ID number of a transmitting port onto a light signal transmitted from an optical demultiplexing unit and the ID number is extracted from the light signal received at a receiving port of an optical multiplexing unit, and the detection of a connection failure occurring in an optical patch cable is carried out for each light amplifier (for example, refer to patent reference 2).